1. Technical Field of the Invention
The present invention relates to a highly temperature-stable current generator. More particularly, the invention relates to a current generator which operates even at low supply voltages (&lt;2.0 v) and with currents in the .mu.A range. The operation of the current generator depends on the characteristics of the technology and can be operable at supply voltages of 1.0 v.
2. Background of the Invention
Conventional design of circuits operating at ever lower voltages (below 2.0 v) required by battery-powered applications poses the problem of producing, in CMOS technology, a highly temperature-stable and/or temperature-insensitive current generator which is capable of producing a reference current in the .mu.A range (in order to have low consumption) with a supply voltage of approximately up to 1.8-2.0 v.
In particular, such current generators are employed in the circuits of nonvolatile memories, such as memory cells of the EEPROM type, to read the content of memory cells (current-driven reading), and in other analog support blocks.
In integrated circuits, analog circuits are usually biased by using conventional current generators, wherein the reference is independent of the supply voltage.
For example, as shown in FIGS. 1 to 4, the following current generators are known: generators which utilize the difference in threshold voltage, .DELTA.Vth, between enhancement-type and native-type (low threshold) transistors (FIG. 1), in which the current reference is given by I=.DELTA.Vth/R; generators which utilize the thermal voltage V.sub.T =KT/q (FIG. 2, where the transistors M1 and M2 have different sizes) ; generators which utilize the overdrive difference between two transistors of the same type (transistors M1, M2 in FIG. 2) having different channel width-to-length (W/L) ratios, for which I=.DELTA.Vov/R (FIG. 3); and generators which generate a reference current based on the voltage across the base and the emitter of a bipolar transistor 20 (FIG. 4), so that the current is equal to I=Vbe/R, where Vbe is the base-emitter voltage of the bipolar transistor 20.
In all the above-cited cases, the resulting current references vary with temperature, due to the fact that the resistance of the resistor and the reference voltage corresponding thereto vary according to this parameter.
The variations exhibited by such current references are on the order of 10% or more.